Stream salmonids as opportunistic foragers: the importance of terrestrial invertebrates along a stream-size gradient

Abstract

Terrestrial invertebrates have been reported to be positively selected by stream salmonids. We assessed the importance of terrestrial and aquatic invertebrates to salmonid diets in 25 streams in Finland, with the hypothesis that terrestrial prey would be important in only the smallest forest streams. Several measures of prey availability were used, including proportional abundance in benthic or drift samples, compared with a trait-based approach, to predict diet composition. Across all 25 streams in autumn, blackfly and caddis larvae were the most important prey items. Terrestrial invertebrates were of moderate importance in all streams, including the smallest. Pure availability predicted diet best and provided, in most cases, a significant fit with the observed diet. In a quantitative literature review, the mean proportion of terrestrial prey in salmonid diets was 17%, being highest for the largest fish (≥15 cm). Species of the genus Salmo consumed significantly less terrestrials than did other salmonid genera. The proportion of terrestrial prey was highest in streams flowing through deciduous forests, but it was only weakly correlated with channel width.